The present invention relates to modular electronic equipment such as computers or communication systems, which need to achieve a maximum of connectivity and more particularly, to a system for precisely positioning a plurality of electronic cards with the aim of plugging them into a backplane.
In most instances, large electronic systems are made of a set of electronic cards providing internal and peer to peer connection through a backplane. With the electronic technology progress, it is often interesting to enhance the performance and the function of an electronic system by adding cards. One or more daughter cards can be connected on a mother card or additional cards can be directly connected to the backplane. This process has been widely deployed across the industry to save development efforts and preserve customers investments. However, because the need of connecting additional cards to a backplane cannot always be foreseen during the design or installation phase, the upgrading of large electrical systems may lead to some mechanical problems in particular for mating cards and backplane connectors.
FIG. 1, FIG. 2 and FIG. 3 describe the background art related to electronic cards and backplane connectors.
FIG. 1 is a perspective view of an electronic module (100) comprising a plurality of cards (102) connected to a backplane (101) according to prior art. The electronic module (100) comprises a mechanical housing (103), one or more mother board cards (102) or Printed Circuit Boards (PCB).The mother cards (104) directly connected to the backplane, and daughter cards (105) connected to mother cards and/or directly to the backplane (107).
The backplane (101) comprises one or a plurality of connectors (106) for receiving said one or plurality of cards equipped with connectors. Mother cards (104) are connected peer to peer through the back plane. Daughter cards (105) are connected either internally on mother cards or directly through the back plane.
FIG. 2 is a section view of the system according to prior art showing two particular embodiments, the first being with the daughter card (201) directly plugged (via connector 208) on an associated mother card (202) which is connected to the backplane (200). The second embodiment comprises a daughter card (203) connected to its mother card through the backplane (200).
A set of spacers (205, 206) is installed between cards (201, 202, 203, 204) to guarantee the mechanical rigidity of the electrical module, the parallel positioning of the electronic cards, and the spacing between the card connectors (207).
FIG. 3 is a section view of the system described in FIG. 1. The electronic module (300) comprises within its mechanical housing (310), a plurality of electronic cards with connectors. Three cards are shown, mother cards (302, 303) and daughter card (304) plugged on mother card (303) by means of connector (305). Cards are stacked together using spacers (S1, S2, S3 . . . ). In general, the first card (303) is fastened to the housing (310) by means of one or a plurality of posts (308) or equivalent means. This first card is generally used as xe2x80x9creference cardxe2x80x9d (309) for positioning the other cards. The problem raised by the prior art is related to the plugging of the electronic module on the backplane (301) and more particularly to the alignment of the card connectors (306) with the backplane connectors (307) taking into account the mechanical tolerances of the various elements.
To evaluate the mechanical tolerance one can refer to FIG. 3 where the spacing between backplane connectors is defined by A1, A2, An, and the spacing between card connectors is defined by X1, X2, Xn. To simplify the description, it will be supposed that:
A1=A2=An
X1=X2=Xn
It will be also assumed that tolerances on connectors are insignificant and negligible (close to zero).
For aligning electronic cards connectors with backplane connectors, a reference card and reference planes must be defined. The first electronic card (303) can be defined as xe2x80x9creference cardxe2x80x9d (309) (in general, the card fastened to the housing is chosen as reference card). For each card, the side where the card connector is located is defined as xe2x80x9creference planexe2x80x9d (R1, R2, R3). There is one reference plane per card. The reference planes in a module comprising a plurality of cards are arranged parallel and are generally oriented towards the same direction. In prior art, the first spacer (S1) is fixed between the reference plane (R1) of the first card and the opposite side of the second card (304).
From a theoretical standpoint we have the following equations.
A1=X1,A2=X2,An=Xn
From an industrial and realistic standpoint, industrial tolerances of the various mechanical elements must be taken in account. The equation becomes:
X1min less than X1 less than X1max
where
X1min=X1xe2x88x92x1 and X1max=X1+x1
with
X1max=S1max+T2max=(S1+s1)+(T2+t2)
X1min=S1min+T2min=(S1xe2x88x92s1)+(T2xe2x88x92t2)
where:
S1, S2, Sn: dimension of spacers 1, 2, n
T1, T2, Tn: thickness of electronic cards (Printed Circuit Boards PCB) 1, 2, n
s1, s2, sn: tolerance on spacer dimension 1, 2, n
t1, t2, tn : tolerance on card thickness 1, 2, n
S1max=S1+s1
S1min=S1xe2x88x92s1
T2max=T2+t2
T2min=T2xe2x88x92t2
The spacing tolerance on second card connector is equal to:
x1=s1+t2
For the third connector, the equation becomes:
X1min+X2min less than X1+X2 less than X1max+X2max
with
X1max+X2max=(S1max+T2max)+(S2max+T3max)
X1min+X2min=(S1min+T2min)+(S2min+T3min)
The spacing tolerance on third card connector is equal to:
x1+x2=s1+s2+t2+t3
The spacing tolerance on the connector of the nth electronic card is equal to:
x1+x2+ . . . +xn=(s1+s2+ . . . +sn)+(t2+t3+ . . . +t(n+1)).
The spacing and thickness tolerances are cumulative, they must be added with each additional card. It becomes more of a problem for systems using a larger number of electronic cards.
This problem can be illustrated with an example. The commercial tolerance for the spacing of the backplane connectors A1, A2, An is +/xe2x88x920.025 mm and the tolerance range is 0.05 mm.
The regular industry thickness tolerance for a Printed Circuit Card (PCB) is 10% with a nominal tolerance of +/xe2x88x925%. The current nominal thickness used nowadays in most industries for ongoing developments is 2.2 mm (+/xe2x88x920.11 mm).
The regular tolerance on commercial spacers S1, S2, Sn is +/xe2x88x920.05 mm and the tolerance range is 0.1 mm. This tolerance is generally given in millimetres and is not function of the spacers length or dimension.
Applying numbers to the equations:
X1max=(S1+0.5)+(T2+0.11)
x1 upper tolerance is 0.16 mm.
X1min=(S1xe2x88x920.05)+(T2xe2x88x920.11)
x1 lower tolerance is 0.16 mm.
X1 tolerance is equal to +/xe2x88x920.16 mm, and the tolerance range is 0.32 mm.
This result must be compared to the tolerance on the spacing between backplane connectors. If we call a1, a2, an: the tolerance on spacing between backplane connectors 1, 2, n and assume:
x1=0.32 mm
a1=0.05 mm
The ratio x1/a1 between tolerance is 6.4:
x1/a1=0.64 mm
With such a ratio, trying to match an electronic module comprising two cards with a backplane will result in damaging connectors contacts. If the electronic module is upgraded with a third card, the situation will be worst because tolerances are cumulative. Thus, the analysis of tolerances shows the limit of the background art when cards stacked in a module have to be plugged at the same time into a backplane.
An alternative for aligning the electronic cards is to attach the cards to the housing body itself. At this point several technologies have to be addressed, such as sheet metal regularly used in the IT (Information Technology) industry, die-casting or full machining.
Considering sheet metal, this technology provide very affordable costs in terms of parts manufacturing, low cost material and tooling associated to minor post process actions such machining, but the problem is the tolerance strength. In facts this technology and the side technologies used to fasten or fix together several items (spot welding or riveting) do not provide the required precision for aligning electronic cards in order to connect at the same time one or more cards.
Considering other technologies such as die-casting or full machining (really not used in the IT industry), if the precision requirements can be achieved, in an economical point of view, these manufacturing processes are time and material consuming. As a result, if those technologies were to be used to produce the parts aimed to hold the electronic cards, the manufacturing cost would be so high that the solution would be abandoned.
The present invention relates to electronic systems such as computers or communication systems which need to achieve a maximum of connectivity using electronic cards for peer to peer connection through a backplane. The invention relates also to electronic modules connected to the backplane of the electronic systems and comprising more than one electronic card stacked in the mechanical housing of the electronic module. All these equipment have a need for a system for precisely positioning a plurality of cards with the aim of mating them exactly into a backplane. A further object of the invention is to provide this alignment of electronic cards even if some cards are missing or are not installed in the module they belong to.
The system for positioning the electronic cards comprises a linking axis for linking cards together comprising parts of different sections including at least one spacing element. It also comprises a reference plane associated to each card according to a predetermined position. The differences of section between the different parts of the linking axis form positioning shoulders aimed at being in contact with the reference planes of the cards. It also comprises maintaining pieces to maintain the reference plane of each card against the associated positioning means. Each card comprises a hole for inserting the linking axis and the surface element around the hole is in contact with the edge of the shoulder associated with the card. The reference planes are oriented towards a same direction. The linking axis is of increasing section, and this section is preferably round. The maintaining pieces comprise for each card a thread on the linking axis, and a nut screwed on the thread for pressing flat the reference plane of the card against the associated positioning elements. The positioning elements comprise in one end a head aimed at being in contact with the reference plane of a last card. The linking axis can be made of a single piece or of a plurality of different elements, each spacing two electronic cards, and in this case, the positioning elements comprise one head located at each end of said different element of said linking axis. Cards are arranged approximately parallel and aimed at being plugged perpendicularly to the back plane. The spacing between cards corresponds approximately to the spacing of the backplane connectors.